Internal gas and liquid distributor for electrodeionization device

ABSTRACT

The present invention provides a resin-wafer electrodeionization (RW-EDI) apparatus including cathode and anode electrodes separated by a plurality of porous solid ion exchange resin wafers, which when in use are filled with an aqueous fluid. The apparatus includes one or more wafers comprising a basic ion exchange medium, and preferably includes one or more wafers comprising an acidic ion exchange medium. The wafers are separated from one another by ion exchange membranes. The gas and aqueous fluid are introduced into each basic wafer via a porous gas distributor which disperses the gas as micro-sized bubbles laterally throughout the distributor before entering the wafer. The fluid within the acidic and/or basic ion exchange wafers preferably includes, or is in contact with, a carbonic anhydrase (CA) enzyme or inorganic catalyst to facilitate conversion of bicarbonate ion to carbon dioxide within the acidic medium.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of U.S. Provisional Application Ser. No. 61/609,705, filed on Mar. 12, 2012, which is incorporated herein by reference in its entirety.

CONTRACTUAL ORIGIN OF THE INVENTION

The United States Government has rights in this invention pursuant to Contract No. DE-AC02-06CH11357 between the United States Government and UChicago Argonne, LLC representing Argonne National Laboratory.

FIELD OF THE INVENTION

This invention relates to removal of carbon dioxide from a gas stream. More particularly, the invention relates to a resin-wafer electrodeionization (EDI) apparatus and methods for removing carbon dioxide from a gas stream, such as a flue gas.

BACKGROUND OF THE INVENTION

Efficient capture and removal of carbon dioxide from flue gas is an essential technology for the development of more environmentally acceptable fossil fuel-based energy production systems, particularly for cleaner coal-based energy production. Several existing carbon dioxide capture processes utilize a vacuum to pull CO₂ across a membrane or from a solvent, which imparts a high energy cost for the CO₂ removal. Others utilize a temperature elevation to drive CO₂ from the solvent. CO₂ capture is the single most costly step in greenhouse gas (GHG) control. Capture of CO₂ from pulverized coal power plant flue gas is crucial for improving the environmental profile of the power generation industry. This represents a principal hurdle for commercializing new fossil fuel-based energy generation technologies in the present, CO₂ constrained world. Key factors in developing new CO₂ capture technologies include cost efficiency, and low capture energy consumption, preferably suitable for use with the relatively low pressure and dilute characteristics of flue gas feed streams. Improved CO₂ capture, driven by the pH dependent equilibrium between gaseous CO₂ and bicarbonate ion, provides a promising avenue for new capture technologies.

Electrodeionization (EDI), also known as electrochemical ion-exchange or continuous deionization, is an advanced ion-exchange technology that combines the advantages of ion-exchange column techniques and electrodialysis. In an EDI process, ion exchange resins are sequestered in dilute compartments to increase ionic conductivity, so that even with very dilute ionic feeds (e.g., 10⁻⁵ N), a stable operation with higher flux and lower energy consumption than electrodialysis becomes possible. EDI technology is presently used to make deionized water for boiler feed and high purity and industrial water applications. A particularly useful variant of EDI (referred to herein as “resin-wafer EDI”) utilizes porous solid ion exchange resin wafers in place of the traditional column or bed of ion exchange beads. There are a number of known EDI apparatus and processes, some of which are described in patent publications and/or issued patents filed on behalf of Argonne National Laboratory (ANL) such as, for example, U.S. Pat. No. 6,797,140, U.S. Pat. No. 6,495,014, U.S. Pat. No. 7,306,934, U.S. Pat. No. 7,141,154, U.S. Patent Publication No. 2008/0187902, U.S. Pat. No. 7,452,920, and U.S. Pat. No. 7,507,318. The entire disclosure of each of the foregoing patents and publications is incorporated herein by reference.

U.S. Patent Publication No. 2010/0300894 to Lin et al., which is incorporated herein by reference in its entirety, describes a resin-wafer EDI apparatus that facilitates removal of CO₂ from a gas stream at a significantly decreased energy consumption compared to the currently used technologies. This apparatus comprises cathode and anode electrodes separated by a plurality of porous solid ion exchange resin wafers, which when in use are filled with an aqueous fluid. The plurality of wafers comprises one or more basic wafers arranged in a stack between the cathode and the anode. The wafers, anode, and cathode are interleaved with ion exchange membranes. Each basic wafer comprises a porous basic ion exchange medium. Each basic wafer is adapted to (a) introduce a CO₂-containing gas into an aqueous fluid within the basic ion exchange medium to convert CO₂ from the gas into bicarbonate ion, and (b) vent a CO₂-depleted gas therefrom. In use, CO₂ is converted to bicarbonate in the fluid within the wafer under the basic conditions of the basic ion exchange medium. The bicarbonate-containing fluid can then be transported out of the apparatus as a concentrated bicarbonate ion solution.

One drawback of the EDI design provided by US2010/033894 is that CO₂ gas entering the wafer sometimes can channel, leading to uneven distribution of the gas in the wafer. This channeling, when it occurs, reduces the efficiency of CO₂ removal. Consequently, there is a need for an improved EDI design that ameliorates CO₂ gas channeling. The present invention addresses this need.

SUMMARY OF THE INVENTION

The present invention provides an improved resin-wafer electrodeionization (RW-EDI) apparatus for removing carbon dioxide from a gas stream with reduced gas channeling relative to the apparatus of Lin et al. described above. The apparatus of the present invention comprises cathode and anode electrodes separated by a plurality of porous solid ion exchange resin wafers, which when in use are filled with an aqueous fluid. The plurality of wafers comprises one or more basic wafers arranged in a stack between the cathode and the anode. The wafers, anode, and cathode are interleaved with ion exchange membranes. Each basic wafer comprises a porous basic ion exchange medium to convert CO₂ from the gas into bicarbonate ion and is adapted to vent a CO₂-depleted gas therefrom.

A porous gas and liquid flow distributor is positioned at the inlet of each wafer. The distributor comprises a porous foam or sintered material positioned across the inlet of each wafer, such that liquid and gas entering the distributor are dispersed laterally through the porous material before entering the resin wafer. The gas is dispersed into micro-sized bubbles by the distributor. The lateral dispersion of the gas bubbles in the porous distributor ameliorates gas channeling after the liquid and gas bubble mixture enters into the wafers. The formation of micro-sized gas bubbles also helps to improve CO₂ capture by increasing the liquid-gas contact surface area. For example, 100 micrometer diameter gas bubbles have a surface area approximately 200 times greater than the surface area of 600 micrometer diameter gas bubbles.

In some embodiments, an additional porous gas distributor can be included intermediate to the inlet and outlet of the wafer, e.g., dividing the wafer into two portions. Again, gas and liquid entering the intermediate porous distributor disperses the gas and liquid laterally, so that the gas will remain evenly distributed in the next portion of the wafer.

The porous gas distributors can be included as separate pieces that are arranged in close contact across the inlet end of the wafer or between wafer portions in the case of an intermediate distributor. Alternatively, the porous distributors can be integral with (e.g., fused to) the wafers. In preferred embodiments, the porous distributors are formed from a porous polymeric foam (e.g., polyurethane or polyethylene foam).

In use, a CO₂ gas is admitted into the porous gas distributor at the inlet of the wafer along with a stream of the aqueous fluid. The aqueous fluid and gas are laterally dispersed and mixed together as they pass through the distributor and into the wafer. The porous distributor disperses the gas into micro-sized bubbles, and creates an even “plug-flow” distribution in the aqueous fluid carrier entering the wafer. CO₂ is converted to bicarbonate in the fluid within the wafer under the basic conditions of the basic ion exchange medium. The bicarbonate-containing fluid can then be transported out of outlet of the wafer as a concentrated bicarbonate ion solution. In one preferred embodiment, the basic ion exchange medium of each wafer contains a carbonic anhydrase enzyme or inorganic catalyst to facilitate conversion of gaseous CO₂ into bicarbonate ion. If desired the carbonic anhydrase enzyme or inorganic catalyst can be chemically or biochemically bound to the basic ion exchange medium thereof to facilitate conversion of gaseous CO₂ into bicarbonate ion. The basic pH value of the basic ion exchange medium is maintained by applying an electric potential appropriate to achieve the desired current (e.g., commonly about 1 to 6 volts per cell pair) across the cathode and anode to provide a driving force for generation and transport of protons (H⁺) toward the direction of the cathode and hydroxyl ions (OH⁻) toward the direction of the anode through the wafers and ion exchange membranes. The transport of protons and hydroxyl ions is balanced with the flow of other cations and anions present in the fluid within the ion exchange media of the wafer to maintain the pH in each wafer or portion in its basic state.

In one preferred embodiment, the wafers are arranged in pairs comprising one basic and one acidic wafer. Each basic wafer comprises a porous basic ion exchange resin medium and each acidic wafer comprises a porous acidic ion exchange resin medium. The basic and acidic wafers in each pair are in fluid communication with one another. Alternatively, each wafer can include a porous basic ion exchange resin portion and a porous acidic ion exchange resin portion in fluid communication with one another via a channel or passageway defined as the interface between the two portions adapted to vent CO₂-depleted gas from the wafer. The ion exchange media of the wafers preferably include, or are in contact with, either a carbonic anhydrase (CA) enzyme [E.C. 4.2.1.1] (or any analogue thereto) or an inorganic catalyst to facilitate conversion of bicarbonate ion to CO₂ within the acidic wafers or acidic portions and/or the conversion of CO₂ to bicarbonate ion in the basic wafers or basic portions.

The plurality of wafers with their adjoining gas distributors are arranged in a stack, with the cathode at one end of the stack and the anode at the other end of the stack. The cathode, distributor/wafer combinations, and anode are interleaved with ion exchange membranes to facilitate a selective flow of ions between the ion exchange media and the electrodes. Each basic resin wafer or basic portion is equipped with a porous gas distributor adapted disperse the CO₂-containing gas stream laterally across each basic wafer or basic portion to introduce a CO₂-containing gas stream and aqueous fluid within the pores of the basic ion exchange medium in an even plug-flow distribution patter. Each wafer also is adapted to vent a CO₂-depleted gas stream from the basic ion exchange medium. Each acidic resin wafer or acidic portion is adapted to vent gaseous CO₂ out of the acidic ion exchange medium.

In use, any CO₂-containing gas stream (e.g., a flue gas) is introduced into an aqueous fluid at the porous distributor, and the mixture is dispersed laterally within the pores of the distributor and then into the pores to the basic ion exchange medium. The gas and aqueous fluid carrier are introduced into the distributor, e.g., through a gas inlet or via a series of microporous membranes or cast channels or other gas delivery method. The CO₂ in the gas stream reacts with the basic ion exchange medium of the wafer to form bicarbonate ion. A CO₂-depleted gas is then vented from the gas outlet of the basic wafer or basic portion. Bicarbonate-containing fluid circulates from the basic ion exchange medium into the acidic ion exchange medium via a fluid passageway between the basic and acidic wafers or portions thereof, where the acid environment converts bicarbonate to gaseous CO₂. The generated CO₂ gas is then vented out of the acidic wafer or acidic portion, e.g., via microporous membranes, channels, or via passing through the pores in and around the ion exchange resin or resin wafer per se.

The conversion between gaseous CO₂ and bicarbonate and/or between bicarbonate and gaseous CO₂ preferably is facilitated by inclusion of a CA enzyme or inorganic catalyst in the aqueous fluid or attached within the pores of the acidic and/or basic ion exchange medium. The appropriate pH values of the basic and acidic ion exchange media are maintained by applying an electric potential appropriate to achieve the desired current (e.g., commonly about 1 to 6 volts per cell pair) across the cathode and anode to provide a driving force for generation and transport of protons (H⁺) toward the direction of the cathode and hydroxyl ions (OH⁻) toward the direction of the anode through the wafers and ion exchange membranes. The transport of protons and hydroxyl ions is balanced with the flow of other cations and anions present in the fluid within the ion exchange media of the wafer to maintain the pH in each wafer or portion in its basic or acidic state, as the case may be. This electrochemical pH control mechanism maintains the proper equilibrium between CO₂ and bicarbonate in each wafer or wafer portion. The flow of gas into the basic ion exchange medium provides a force that directs fluid flow from the basic ion exchange medium into the acidic ion exchange medium and then back again in a circular fashion.

In practice, a prototype of the apparatus of the present invention has achieved nearly 90 volume percent removal of carbon dioxide from a flue gas stream. In addition the prototype device produced carbon dioxide purity approaching 100%.

In one preferred embodiment of the apparatus of the present invention, the cathode and anode are separated by a plurality of alternating porous wafer pairs. Each pair comprises a porous basic ion exchange resin wafer and a porous acidic ion exchange resin wafer separated by a cation exchange membrane (CEM). Each wafer pair is separated from any adjacent wafer pair by an ion exchange membrane, preferably a bipolar membrane (BPM). The cathode is positioned adjacent a basic resin wafer, and separated therefrom by an ion exchange membrane, preferably a BPM. The anode is positioned adjacent an acidic resin wafer and separated therefrom by an ion exchange membrane, preferably a BPM. In this arrangement, bicarbonate circulates within a given pair of wafers but not between different wafer pairs.

In an alternative embodiment, each wafer includes a porous basic ion exchange resin portion at one end and a porous acidic ion exchange resin portion at the other end thereof. The basic and acidic portions are in fluid communication with one another by a channel containing hollow fiber tubes adapted to act as the gas outlet of the basic portion of the wafer. A porous gas distributor is positioned across the inlet end of each basic portion of the wafer to evenly distribute the gas and fluid entering the wafer. In this configuration, each wafer preferably is separated from adjacent wafers by a passageway defined by an anion exchange membrane (AEM) adjacent to one wafer and a CEM adjacent to the next wafer of the stack (an AEM-CEM pair) in the direction from cathode to anode. Each passageway is in fluid communication with an adjacent wafer such that ion-containing fluid can circulate from the acidic portion of a wafer to the basic portion of the same wafer through the passageway. The cathode is separated from its adjacent wafer by a CEM, and the anode is separated from its adjacent wafer by a passageway defined by an AEM-CEM pair. In this arrangement, bicarbonate circulates between the basic and acidic portions of a given wafer, but not between different wafers, and there is a net flow of protons toward the cathode and hydroxyl ions toward the anode, to provide electrochemical pH control in the basic and acidic resin portions of the wafers.

The present invention could be used to capture CO₂ from flue gas from coal, natural gas, or oil power plants or other combustion or industrial processes to prevent release to the atmosphere. In addition, the present invention could be used to capture/separate CO₂ from either natural gas or biological methane (biogas) to increase the methane content. Thus, the present invention could be used to mitigate greenhouse gas emissions or capture CO₂ for beneficial uses.

The invention consists of certain novel features and a combination of parts hereinafter fully described, illustrated in the accompanying drawings, and particularly pointed out in the appended claims, it being understood that various changes in the details may be made without departing from the spirit, or sacrificing any of the advantages of the present invention.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic illustration of one embodiment of the apparatus of the present invention, in which the plurality of wafers are arranged in pairs, with one basic wafer and one acidic wafer in each pair.

FIG. 2 is a schematic illustration of an embodiment of the apparatus of the present invention, in which the each wafer includes a basic ion exchange medium portion and an acidic ion exchange medium portion.

FIG. 3 schematically illustrates an alternative embodiment of the apparatus of FIG. 1, which includes an intermediate porous gas distributor 21 b in the approximate middle of the wafer 16.

DETAILED DESCRIPTION OF THE PRESENT INVENTION

The present invention provides a resin-wafer electrodeionization (RW-EDI) apparatus for removing carbon dioxide from a gas stream. In one aspect, the RW-EDI apparatus comprises a cathode, and an anode that is separated from the cathode by a plurality of porous solid ion exchange resin wafers. The wafers are arranged in a stack between the cathode and the anode. The stack preferably comprises one or more basic wafers and a number of acidic wafers equal to the number of basic wafers. The wafers are arranged in pairs of basic and acidic wafers, with the basic wafer of a pair being in fluid communication with the acidic wafer of the same pair. Alternatively, each wafer can comprise a basic portion in fluid communication with an acidic portion of the same wafer.

Preferably, the pH of fluid within the basic ion exchange medium of the wafers is maintained in the range of about 7.5 to about 9.5, and the pH of fluid within the acidic ion exchange medium, when present, is maintained in the range of about 6 to about 7, by the electric potential applied across the cathode and the anode. Typically, the electric potential applied across the anode and cathode is at least about 1 Volt (e.g., about 1 to 6 Volts in this example) per cell pair for the desired current, although up to about 12 V per cell pair can be applied, for a total voltage of about 12V-times the total number of cell pairs +about 5 V. The actual voltage per cell pair will vary with the thickness of the wafers, as will be evident to those of ordinary skill in the art. Thicker wafers will require higher voltages. Preferably, the voltage per cell pair will be greater than 1 V if the wafer thickness is greater than 1 mm. Typically, the wafers will have a thickness in the range of about 1 mm to about 20 mm. However, greater wafer thickness may still work. The determination of an appropriate working voltage is within the level of ordinary skill in the art.

Each basic wafer or basic portion contains a porous basic ion exchange medium and each acidic wafer or acidic portion contains a porous acidic ion exchange medium. A primary porous gas distributor is positioned across the inlet end of each wafer. The primary distributor comprises a porous polymeric foam or sintered material positioned in close contact with or integral with the inlet end of each basic wafer, such that liquid and gas entering the distributor are dispersed laterally through the porous material of the distributor before entering the resin wafer. The lateral dispersion of the gas in the porous distributor provides an even plug-flow distribution of the gas and fluid, which reduces gas channeling in the wafers during operation of the EDI apparatus. In some embodiments, an additional intermediate porous gas distributor can be included intermediate to the inlet and outlet ends of the wafer, dividing the wafer into two portions (e.g., in approximately the middle of the wafer). Gas and liquid entering the intermediate porous distributor disperses the gas and liquid laterally, so that the gas will remain evenly distributed in the next portion of the wafer.

The porous gas distributors can be included as separate pieces that are arranged in close contact across the inlet end of the wafer or between wafer portions in the case of an intermediate distributor. Alternatively, the porous distributors can be integral with (e.g., fused to) the wafers, as described herein. In preferred embodiments, the porous distributors are formed from a porous polymeric foam (e.g., polyurethane or polyethylene foam). Each porous distributor is adapted for introducing CO₂-containing gas bubbles into an aqueous fluid within the basic wafer or portion to direct an even distribution of micro-sized gas bubbles into the basic wafer or basic portion to convert CO₂ from the gas into bicarbonate ion. The bicarbonate then flows into an acidic wafer or acidic portion that is in fluid communication with the basic wafer or basic portion to convert bicarbonate ion into CO₂ gas. As used herein, the terms “micro-sized gas bubbles”, “microbubbles”, and grammatical variations thereof, refer to gas bubbles having a diameter of less than about 1000 micrometers. The size (i.e., volume) of the pores of the gas distributor influences the size of gas bubbles produced therein. Preferably, the porous distributor has an average pore diameter of about 100 to about 600 micrometers. While it is understood that the pores of the gas distributor may not be spherical in shape, for convenience, the phrase “average pore size of about 100 micrometers” refers to pores having a volume equivalent to the volume of a sphere having a diameter of 100 micrometers.

Each basic wafer or basic portion also is adapted to vent a CO₂-depleted gas therefrom; and each acidic wafer or acidic portion is adapted to vent CO₂ gas generated in the acidic wafer or acidic portion. Gas can be introduced into or vented from the wafers or portions thereof via gas inlet and/or outlet tubes, via microporous membranes or channels in contact with or connected to the wafers or portions, or by any other suitable structure. The cathode, wafers with their accompanying distributor(s), and anode are interleaved with ion exchange membranes, and collectively direct a net flow of protons through the wafers toward the direction of the cathode and a net flow of hydroxyl ions through the wafers toward the direction of the anode, when an electric potential is applied to across cathode and anode. The flow of protons and hydroxyl ions, combined with the capture and release of CO₂, maintains a basic pH in each basic wafer or basic portion and an acidic pH in each acidic wafer or acidic portion.

In one embodiment, the plurality of wafers comprises pairs of alternating basic and acidic wafers with a basic wafer and an acidic wafer in each pair, and a cation exchange membrane between the basic and acidic wafers of the pair. The cathode is positioned adjacent to a basic wafer with a bipolar membrane therebetween, while the anode is positioned adjacent to an acidic wafer with a bipolar membrane therebetween. Each pair of wafers is separated from any adjacent pair by a bipolar ion exchange membrane.

In another embodiment, each wafer comprises a basic portion comprising a porous basic ion exchange medium and an acidic portion comprising a porous acidic ion exchange medium in fluid communication with the basic portion at a gas-vented juncture defined at the interface between the basic portion and the acidic portion of the wafer. The primary porous gas distributor is positioned across the inlet end of the basic portion of the wafer. Each wafer is separated from any adjacent wafer by a fluid-flow passageway in fluid communication with the basic and acidic portions of the same wafer. The passageway is defined by a spaced pair of an anion exchange membrane adjacent to one wafer and a cation exchange membrane adjacent to the next wafer in the stack in the direction from cathode to anode. The cathode is separated from an adjacent wafer by a cation exchange membrane, and the anode is separated from an adjacent wafer by a passageway defined by a spaced pair of an anion exchange membrane and a cation exchange membrane. The vented juncture between the basic and acidic portions of each wafer is adapted to vent the CO₂-depleted gas from the wafer. Preferably, the juncture between the basic portion and the acidic portion of each wafer comprises hollow fiber tubes adapted to vent the CO₂-depleted gas from the wafer. Other gas exit (gas/liquid separation) strategies known in the art are equally acceptable.

The acidic ion exchange medium can contain a carbonic anhydrase enzyme or enzyme or catalyst analogues thereto to facilitate conversion of bicarbonate ion to gaseous CO₂. The carbonic anhydrase enzyme or inorganic catalyst can be chemically or biochemically bound to the acidic ion exchange medium, if desired. In addition, or alternatively, the basic ion exchange medium of a wafer can absorb a carbonic anhydrase enzyme or inorganic catalyst to facilitate conversion of gaseous CO₂ to bicarbonate ion. The carbonic anhydrase enzyme or inorganic catalyst also can be chemically or biochemically bound to the basic ion exchange medium, if desired.

In a preferred apparatus aspect, the apparatus comprises a cathode, and an anode that is separated from the cathode by a plurality of porous solid ion exchange resin wafers arranged in a stack between the cathode and the anode. The wafers are arranged in pairs of alternating basic and acidic wafers with a basic wafer and an acidic wafer in each pair, and a cation exchange membrane between the basic and acidic wafers of the pair. The inlet end of each wafer is fitted with a porous gas distributor, as described herein. The cathode is positioned adjacent to a basic wafer with a bipolar membrane therebetween, while the anode is positioned adjacent to an acidic wafer with a bipolar ion exchange membrane therebetween. Each pair of wafers is separated from any adjacent pair by a bipolar ion exchange membrane. The stack of wafers alternates from basic to acidic to basic, etc. in the direction from the cathode to the anode. Each basic wafer comprises a porous basic ion exchange medium and each acidic wafer comprises a porous acidic ion exchange medium. Each gas distributor is adapted for introducing a CO₂-containing gas into an aqueous fluid and dispersing the gas and fluid laterally through the distributor before the gas and fluid mixture enter the basic wafer to convert CO₂ from the gas into bicarbonate ion. The resulting bicarbonate-containing fluid then flows into the acidic wafer of the pair to convert bicarbonate ion into CO₂ gas, as described above. Each basic wafer also is adapted to vent a CO₂-depleted gas therefrom, and each acidic wafer includes a CO₂ outlet to vent CO₂ gas generated in the acidic wafer, as described above.

The cathode, wafer/distributor combinations, cation exchange membranes, bipolar ion exchange membranes, and the anode together direct a net flow of protons through the wafers toward the direction of the cathode and a net flow of hydroxyl ions through the wafers toward the direction of the anode when an electric potential is applied to the cathode and anode. The flow of protons and hydroxyl ions, balanced with the capture and release of CO₂, maintains a basic pH in each basic wafer portion and an acidic pH in each acidic wafer.

The acidic ion exchange medium preferably contains a carbonic anhydrase enzyme or inorganic catalyst to facilitate conversion of bicarbonate ion to gaseous CO₂. The carbonic anhydrase enzyme or inorganic catalyst can be chemically bound to the acidic ion exchange medium, if desired. In addition, or alternatively, the basic ion exchange medium can contain a carbonic anhydrase enzyme or inorganic catalyst to facilitate conversion of gaseous CO₂ to bicarbonate ion, which can be chemically bound to the basic ion exchange medium, if desired.

FIG. 1 schematically illustrates an embodiment of a RW-EDI apparatus of the present invention. Apparatus 10 comprises a cathode 12 and an anode 14, separated by a plurality of porous solid ion exchange resin wafers 16 and 18 arranged in pairs 26 of alternating wafers 16 and 18. Each pair 26 includes a basic wafer 16 containing a porous basic ion exchange medium 17, and an acidic wafer 18 containing an acidic ion exchange medium 19. A gas inlet structure 20 is adapted and configured to introduce a CO₂-containing gas into an aqueous fluid within gas distributor 21. The resulting mixture of CO₂-containing gas bubbles into an aqueous fluid then inters into the basic ion exchange medium 17 of wafer 16. An opposed gas outlet structure 22 is positioned at the opposite end of wafer 16 to vent a CO₂-depleted gas from the wafer. Wafers 16 and 18 of each pair 26 are separated from one another by a cation exchange membrane 28. Each pair 26 is separate from any adjacent pair 26 by a bipolar ion exchange membrane 30, with an acidic wafer 18 of one pair 26 facing a basic wafer 16 of another pair 26. Cathode 12 is positioned adjacent to a basic wafer 16, with a bipolar ion exchange membrane between cathode 12 and wafer 16. Anode 14 is positioned adjacent to an acidic wafer 18, with a bipolar ion exchange membrane between anode 14 and wafer 18.

During use, an electric potential is applied across cathode 12 and anode 14, while a CO₂-containing gas stream (e.g., a flue gas) is introduced into an aqueous fluid within each porous gas distributor 21 via gas inlet structure 20, and then into wafer 16. The basic ion exchange medium 17 converts CO₂ into bicarbonate ion. CO₂-depleted gas is vented from each wafer 16 through outlet structure 22. Bicarbonate-containing aqueous fluid flows from basic wafer 16 into acidic wafer 18 via a fluid passageway 36 between wafer 16 and wafer 18, propelled by the gas stream. The bicarbonate is converted to gaseous CO₂ by acidic ion exchange medium 19 of wafer 18. Each wafer 18 includes a CO₂ outlet structure 24 to vent gaseous CO₂ out of wafer 18. Bicarbonate-depleted fluid flows from wafer 18 back into wafer 16 via an opposed fluid flow passageway 38. Horizontal arrows in FIG. 1 denote the flow of ions within apparatus 10 when in use. The flow of bicarbonate ion-containing aqueous fluid from wafer 16 of a pair 26 into wafer 18 of the same pair 26, and subsequent conversion of the bicarbonate ion to gaseous CO₂ in wafer 18 effectively prevents any net flow of bicarbonate ion from one pair 26 to another pair 26. Bipolar membranes 30, 32, and 34 allow balancing counter ions to flow from one wafer 16 or 18 to another as well as to cathode 12 and anode 14, such that there is a net flow of protons toward the direction of cathode 12 and a net flow of hydroxyl ions toward the direction of anode 14, thereby maintaining a basic pH in basic ion exchange medium 17 and an acidic pH in acidic ion exchange medium 19.

A prototype apparatus of the type schematically shown in FIG. 1 was prepared and tested to determine the evenness of gas flow. The results of this testing indicated that gas channeling observed in a similar apparatus without the gas distributors was prevented and/or greatly diminished in the apparatus of the present invention, which included porous gas distributors 21 across the inlet end of the basic wafers 16. The EDI apparatus with the gas distributors retained the positive benefits present in the apparatus of US2010/0300894, but without the undesirable gas channeling problems associated therewith.

The resin wafers preferably are fabricated from commercial grade resins (e.g., PUROLITE® resins; The Purolite Co.). The resin wafers with different cation/anion mixing ratios were fabricated using the process described in U.S. Pat. Nos. 6,797,140, 7,306,934, and 7,452,920. Commercial polymeric ion-exchange membranes (Ameridia Inc.) were used in assembling the RW-EDI stack. Different membrane configurations were evaluated. A commercial ED stack of 195 cm² membrane size was used as the base template to assemble the EDI stack. Two cell pairs were used for the evaluations. A 10% CO₂ gas (balance N₂) was fed directly into the capture compartments. Solution bicarbonate concentrations were analyzed by a total organic carbon (TOC) analyzer. All the samples were conditioned with NaOH and kept in sealed sample vials to avoid atmosphere exchange of CO₂. Around 3-6 volts were applied in the EDI between cathode and anode. The pH of the basic and acidic chambers were around 8.5 and 6.5, respectively.

The primary and intermediate porous gas distributors can be fabricated with any porous material (e.g., a foam or sintered material) having an open pore structure which allows dispersion of the gas and fluid laterally throughout the distributor. In some preferred embodiments, the gas distributors are fabricated from a porous foam, such as a polyurethane foam.

In another apparatus aspect of the invention the RW-EDI apparatus comprises a cathode, and an anode that is separated from the cathode by a plurality of porous solid ion exchange resin wafers arranged in a stack between the cathode and the anode. Each wafer comprises a basic portion containing a porous basic ion exchange medium and an acidic portion containing a porous acidic ion exchange medium in fluid communication with the basic ion exchange medium at a gas-vented juncture defined at the interface between the basic portion and the acidic portion of the wafer. Each wafer is separated from any adjacent wafer by a fluid-flow passageway in fluid communication with the basic and acidic portions of the same wafer. The passageway is defined by a spaced pair of an anion exchange membrane and a cation exchange membrane. The cathode is separated from an adjacent wafer by a cation exchange membrane, while the anode is separated from an adjacent wafer by a passageway defined by a spaced pair of an anion exchange membrane and a cation exchange membrane.

A gas inlet structure is adapted and configured for introducing a CO₂-containing gas into an aqueous fluid in a porous gas distributor as described above. The dispersed gas bubbles and liquid mixture in the distributor then enters the basic ion exchange medium to convert CO₂ from the gas into bicarbonate ion. The bicarbonate then flows into the acidic portion to convert bicarbonate ion into CO₂ gas. A channel or passageway defined at the interface between the basic and acidic portions of the wafer vent CO₂-depleted gas from the wafer. The acidic portion of each wafer includes a CO₂ outlet structure to vent CO₂ gas generated therein. For example, the interface between the basic portion and the acidic portion of each wafer comprises hollow fiber tubes adapted to vent the CO₂-depleted gas from the wafer. Other methods can be used to direct the gas away from the wafer. A preferred method for removing the CO₂ depleted gas comprises inverted cones positioned adjacent the wafers, like those used to remove product gas in some anaerobic digesters.

The cathode, gas distributors, wafers, cation exchange membranes, anion exchange membranes, and the anode together direct a net flow of protons through the wafers toward the direction of the cathode and a net flow of hydroxyl ions through the wafers toward the direction of the anode when an electric potential is applied across the cathode and anode, to thereby maintain a basic pH in each basic wafer portion and an acidic pH in each acidic wafer.

Preferably, the acidic ion exchange medium and/or the basic ion exchange medium contains a carbonic anhydrase enzyme [E.C. 4.2.1.1] (or any analogue thereto) or inorganic catalyst to facilitate conversion of bicarbonate ion to gaseous CO₂ and/or the conversion of CO₂ to bicarbonate ion, respectively. The carbonic anhydrase enzyme or inorganic catalyst can be chemically or biochemically bound to the acidic and/or basic ion exchange medium, if desired.

FIG. 2 illustrates another embodiment of a RW-EDI apparatus of the present invention. Apparatus 100 comprises a cathode 112 and an anode 114, separated by a plurality of porous solid ion exchange resin wafers 115. Each wafer 115 includes an acidic portion 116 and an opposed basic portion 118. Each basic portion 116 contains a porous basic ion exchange medium 117, and each acidic portion 118 contains an acidic ion exchange medium 119. A gas inlet structure 120 feeds gas and aqueous fluid into porous gas distributor 121, to disperse the gas in the fluid and laterally distribute the resulting mixture of gas bubbles and fluid across the distributor for introducing gas/fluid mixture into the basic ion exchange medium 117 of basic portion 116. Juncture 122 between basic portion 116 and acidic portion 118 of each wafer 115 acts as a gas outlet to vent a CO₂-depleted gas from the wafer, e.g., through hollow fibers 123.

Each acidic portion 118 includes a CO₂ outlet structure 124 to vent gaseous CO₂ out of portion 118. Each wafer 115 is separate from any adjacent wafer 115 by a fluid flow passageway 129 defined by a spaced pair of an anion exchange membrane 128 and a cation exchange membrane 130, arranged in the same order throughout the apparatus in the direction from cathode 112 to anode 114. Cathode 112 is separated from its adjacent wafer 115 by a cation exchange membrane 132. Anode 114 is separated from its adjacent wafer 115 by a passageway 129 defined by a spaced pair of an anion exchange membrane 128 and a cation exchange membrane 130, with membrane 130 being immediately adjacent to anode 114.

During use, an electric potential is applied across cathode 112 and anode 114, while a CO₂-containing gas stream (e.g., a flue gas) is introduced into an aqueous fluid within each distributor 121 via gas inlets 120, and then into each adjacent wafer 115. Distributor 121 laterally disperses the gas and fluid before directing the mixture of gas and fluid into basic portion 116. Basic ion exchange medium 117 converts CO₂ into bicarbonate ion. CO₂-depleted gas is vented from each wafer 115 through outlet juncture 122 (e.g., via hollow fiber tubes 123). Bicarbonate-containing aqueous fluid flows from basic wafer portion 116 into acidic wafer portion 118 via juncture 122, propelled by the gas stream. The bicarbonate is converted to gaseous CO₂ by the acidic ion exchange medium 119 of portion 118, and bicarbonate-depleted fluid flows from portion 118 back into portion 116 via passageway 129. Horizontal arrows in FIG. 2 denote the flow of ions within apparatus 100 when in use. The flow of bicarbonate ion-containing aqueous fluid from basic portion 116 into acidic portion 118 of the same wafer 115, and subsequent conversion of the bicarbonate ion to gaseous CO₂ in acidic portion 118 effectively prevents any net flow of bicarbonate ion from one wafer 115 to another wafer 115. Ion exchange membranes 128, 130, and 132 allow balancing counter ions to flow from one wafer 115 to another wafer 115, as well as to cathode 112 and anode 114, such that there is a net flow of protons toward the direction of cathode 112 and a net flow of hydroxyl ions toward the direction of anode 114, thereby maintaining a basic pH in basic ion exchange medium 117 and an acidic pH in acidic ion exchange medium 119. The thickness or other dimensions of the adsorption (basic wafer) and desorption (acidic wafer) cells could be different to accommodate different rates of adsorption and desorption.

FIG. 3 illustrates an alternative embodiment of EDI apparatus 10 of FIG. 1. In this embodiment, EDI apparatus 10 b includes an intermediate porous gas distributor 21 b in the approximate middle of the wafer 16.

In another format, the gas adsorption portion of the apparatus is used without a gas desorption portion. In this format CO₂ gas is removed from a mixed gas stream and be available as a concentrated bicarbonate solution. Bicarbonate solutions have applications such as enhancing algae production or for other CO₂ uses.

A method aspect of the present invention comprises applying an electric potential across the cathode and anode of RW-EDI apparatus of the invention, and introducing a CO₂-containing gas into an aqueous fluid within the pores of the primary gas distributor and into the basic ion exchange medium within each basic wafer or basic portion of a wafer of the apparatus through the gas inlet thereof, venting CO₂-depleted gas from the gas outlet of each basic wafer or basic portion, and venting gaseous CO₂ from the CO₂ outlet of each acidic wafer or acidic portion of a wafer. In some preferred embodiments, the gaseous CO₂ is collected for later use or disposal. Preferably, the electric potential applied across the cathode and the anode is in the range of about 1 to 6 volts per cell pair. The CO₂ gas and/or the CO₂-depleted gas can be vented at atmospheric pressure. Preferably, the pH of fluid within the basic ion exchange medium of the wafers is maintained in the range of about 7.5 to about 9.5, and the pH of fluid within the acidic ion exchange medium is maintained in the range of about 6 to about 7, by the electric potential applied across the cathode and the anode.

Desorption of CO₂ from bicarbonate under acidic conditions is a crucial step in the CO₂ removal process, because it is not as kinetically favored as the absorption of CO₂ to form bicarbonate ion under basic conditions. Because of this kinetic difference, it is preferable to include a carbonic anhydrase enzyme or inorganic catalyst within the aqueous fluid in the ion exchange media, since the CA enzyme or inorganic catalyst catalyzes the rate of conversion of bicarbonate to gaseous CO₂ by a factor of about 3 to 4.

In some conventional CO₂ removal processes a vacuum is used to drive the equilibrium toward gaseous CO₂, however, this adds considerable capital expense and energy costs to such systems. The RW-EDI apparatus of the present invention eliminates the need for vacuum systems, particularly in embodiments that incorporate CA enzyme. Suitable CA enzymes are known in the art, such as for example, those as described in U.S. Pat. No. 6,143,556 (Trachtenberg; incorporated herein by reference in its entirety), U.S. Patent Publication No. 2008/0003662 (Trachtenberg; incorporated herein by reference in its entirety), and the CA enzymes available from Carbozyme, Inc. (Monmouth Junction, N.J.).

In some embodiments, it may be preferable to immobilize the CA enzyme within the ion exchange media of the wafers. In such cases, the enzyme can be immobilized by any suitable method known in the art, such as the method described in U.S. Pat. No. 7,306,934 (Arora et al.; incorporated herein by reference in its entirety).

Porous solid resin wafer ion exchange materials suitable for use in the present invention can be prepared in any suitable manner known in the art, such as for example the materials and methods described in U.S. Pat. No. 6,495,014 (Datta et al., incorporated herein by reference in its entirety), U.S. Pat. No. 7,452,920 (Lin et al., incorporated herein by reference in its entirety), and U.S. Pat. No. 7,306,934 (Arora et al.).

The apparatus and method of the present invention provide a number of benefits, including without limitation, providing for improved, even, plug-flow distribution of gas bubbles in the wafers during operation and providing a purified CO₂ stream from a dilute flue gas. The purified CO₂ obtained with the apparatus and methods of the present invention can conveniently be recovered for use in other industrial processes, if desired. In addition, no vacuum is needed to recover the purified CO₂ from the flue gas, thus reducing energy and capital costs relative to conventional CO₂ removal processes. The apparatus and methods of the present invention also can be readily produced at any desired scale.

The apparatus and method has several useful applications. It can be used to capture CO₂ from flue gas from coal, natural gas, or oil power plants or other combustion or industrial processes to prevent release to the atmosphere. In addition, the present invention could be use to capture CO₂ from either natural gas or biological methane (biogas) to increase the methane content. Thus the present invention could be used to mitigate greenhouse gas emissions. The apparatus of the present claims also can be used to capture other gaseous pollutants, for example from industrial facilities such as coal-fired power plants, e.g., to capture ammonia, sulfur dioxide, and other gaseous pollutant materials using a liquid having a suitable pH (i.e., pH greater than 7 for capture of acidic materials, and pH less than 7 for capture of basic materials) as a scrubbing agent.

All references, including publications, patent applications, and patents, cited herein are hereby incorporated by reference to the same extent as if each reference were individually and specifically indicated to be incorporated by reference and were set forth in its entirety herein.

The use of the terms “a” and “an” and “the” and similar referents in the context of describing the invention (especially in the context of the following claims) are to be construed to cover both the singular and the plural, unless otherwise indicated herein or clearly contradicted by context. The terms “comprising,” “having,” “including,” and “containing” are to be construed as open-ended terms (i.e., meaning “including, but not limited to,”) unless otherwise noted. Recitation of ranges of values herein are merely intended to serve as a shorthand method of referring individually to each separate value falling within the range, unless otherwise indicated herein, and each separate value is incorporated into the specification as if it were individually recited herein. All methods described herein can be performed in any suitable order unless otherwise indicated herein or otherwise clearly contradicted by context. The use of any and all examples, or exemplary language (e.g., “such as”) provided herein, is intended merely to better illuminate the invention and does not pose a limitation on the scope of the invention unless otherwise claimed. No language in the specification should be construed as indicating any non-claimed element as essential to the practice of the invention.

Preferred embodiments of this invention are described herein, including the best mode known to the inventors for carrying out the invention. Variations of those preferred embodiments may become apparent to those of ordinary skill in the art upon reading the foregoing description. The inventors expect skilled artisans to employ such variations as appropriate, and the inventors intend for the invention to be practiced otherwise than as specifically described herein. Accordingly, this invention includes all modifications and equivalents of the subject matter recited in the claims appended hereto as permitted by applicable law. Moreover, any combination of the above-described elements in all possible variations thereof is encompassed by the invention unless otherwise indicated herein or otherwise clearly contradicted by context. 

The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows:
 1. A resin-wafer electrodeionization (RW-EDI) apparatus for removing carbon dioxide from a gas stream, the apparatus comprising: a cathode; and an anode that is separated from the cathode by a plurality of porous solid ion exchange resin wafers arranged in a stack between the cathode and the anode, and comprising pairs of alternating basic and acidic wafers with a basic wafer and an acidic wafer in each pair, and a cation exchange membrane between the basic and acidic wafers of each pair; the cathode being adjacent to a basic wafer with a bipolar membrane therebetween, the anode being adjacent to an acidic wafer with a bipolar ion exchange membrane therebetween, each pair of wafers being separated from an any adjacent pair by a bipolar ion exchange membrane; and each basic wafer contacting a primary porous gas distributor across an inlet end of the basic wafer; wherein: each basic wafer comprises a porous basic ion exchange medium and each acidic wafer comprises a porous acidic ion exchange medium; each primary porous gas distributor is adapted and configured to introduce a CO₂-containing gas into an aqueous fluid as micro-sized gas bubbles and laterally distribute the gas bubbles and fluid throughout the primary porous gas distributor and into the inlet end of the basic wafer in contact therewith; the basic ion exchange medium within each basic wafer is adapted and configured to convert CO₂ from the gas into bicarbonate ion, and the basic wafers are adapted to vent a CO₂-depleted gas therefrom; each acidic wafer is adapted to vent CO₂ gas generated in the acidic ion exchange medium thereof out of the apparatus; and when in use, bicarbonate-containing fluid flows from the basic ion exchange medium into the acidic ion exchange medium in fluid communication therewith to convert bicarbonate ion into CO₂ gas, and the cathode, gas distributors, wafers, cation exchange membranes, bipolar ion exchange membranes, and the anode together are adapted and configured to direct a net flow of protons through the wafers toward the direction of the cathode and a net flow of hydroxyl ions through the wafers toward the direction of the anode when an electric potential is applied across the cathode and anode, to thereby maintain a basic pH in each basic wafer portion and an acidic pH in each acidic wafer during capture and release of CO₂ from the gas stream.
 2. The apparatus of claim 1 wherein the acidic ion exchange medium contains a carbonic anhydrase enzyme or inorganic catalyst to facilitate conversion of bicarbonate ion to gaseous CO₂.
 3. The apparatus of claim 2 wherein the carbonic anhydrase enzyme or inorganic catalyst is chemically bound to the acidic ion exchange medium.
 4. The apparatus of claim 1 wherein the basic ion exchange medium contains a carbonic anhydrase enzyme or inorganic catalyst to facilitate conversion of gaseous CO₂ to bicarbonate ion.
 5. The apparatus of claim 4 wherein the carbonic anhydrase enzyme or inorganic catalyst is chemically bound to the basic ion exchange medium.
 6. The apparatus of claim 1 wherein each wafer has a thickness in the range of about 1 mm to about 20 mm.
 7. The apparatus of claim 6 further comprising an intermediate porous gas distributor between the inlet end and an outlet end of each basic wafer, dividing the basic wafer into two portions.
 8. The apparatus of claim 7 wherein each primary porous gas distributor and intermediate porous gas distributor comprises a porous polymeric foam.
 9. The apparatus of claim 1 wherein each primary porous gas distributor comprises a porous polymeric foam.
 10. The apparatus of claim 1 wherein the porous gas distributor has an average pore size of about 100 to about 600 micrometers.
 11. A resin-wafer electrodeionization (RW-EDI) apparatus for removing carbon dioxide from a gas stream, the apparatus comprising: a cathode; and an anode that is separated from the cathode by a plurality of porous solid ion exchange resin wafers arranged in a stack between the cathode and the anode, each wafer comprising a basic portion containing a porous basic ion exchange medium and an acidic portion containing a porous acidic ion exchange medium, the basic ion exchange medium being in fluid communication with the acidic ion exchange medium of the same wafer at a gas-vented juncture defined at the interface between the basic portion and the acidic portion of the wafer; and each basic portion contacting a primary porous gas distributor across an inlet end of the basic portion; wherein: each wafer is separated from any adjacent wafer by a fluid-flow passageway in fluid communication with the basic and acidic portions of the same wafer, the passageway being defined by a spaced pair of an anion exchange membrane and a cation exchange membrane; the cathode being separated from an adjacent wafer by a cation exchange membrane, and the anode being separated from an adjacent wafer by a passageway defined by a spaced pair of an anion exchange membrane and a cation exchange membrane; each primary porous gas distributor has an average pore size in the range of about 100 to about 600 micrometers, and is adapted and configured to introduce a CO₂-containing gas into an aqueous fluid as gas bubbles having an average diameter in the range of about 100 to about 600 micrometers, and laterally distribute the gas bubbles and fluid throughout the distributor and into the inlet end of the basic portion of the wafer in contact therewith; the basic ion exchange medium in the basic portion of each wafer is adapted to convert CO₂ from the gas into bicarbonate ion, which then flows into the acidic ion exchange medium of the acidic portion of the same wafer to convert bicarbonate ion into CO₂ gas; the vented juncture between the basic and acidic portions venting CO₂-depleted gas from the wafer; and the acidic portion of each wafer is adapted to vent CO₂ gas generated in the acidic ion exchange medium thereof out of the apparatus; and when in use, the cathode, gas distributors, wafers, cation exchange membranes, anion exchange membranes, and the anode together direct a net flow of protons through the wafers toward the direction of the cathode and a net flow of hydroxyl ions through the wafers toward the direction of the anode when an electric potential is applied across the cathode and anode, to thereby maintain a basic pH in the basic ion exchange medium of each wafer each basic and an acidic pH in the acidic ion exchange medium of each wafer during capture and release of CO₂ from the gas stream.
 12. The apparatus of claim 11 wherein the vented juncture between the basic portion and the acidic portion of each wafer comprises hollow fiber tubes adapted to vent the CO₂-depleted gas from the wafer.
 13. The apparatus of claim 11 wherein the acidic ion exchange medium of each wafer contains a carbonic anhydrase enzyme or inorganic catalyst to facilitate conversion of bicarbonate ion to gaseous CO₂.
 14. The apparatus of claim 13 wherein the carbonic anhydrase enzyme or inorganic catalyst is chemically bound to the acidic ion exchange medium.
 15. The apparatus of claim 11 wherein the basic ion exchange medium of each wafer contains a carbonic anhydrase enzyme or inorganic catalyst to facilitate conversion of gaseous CO₂ to bicarbonate ion.
 16. The apparatus of claim 15 wherein the carbonic anhydrase enzyme or inorganic catalyst is chemically or biochemically bound to the basic ion exchange medium.
 17. The apparatus of claim 11 wherein each wafer has a thickness in the range of about 1 mm to about 20 mm.
 18. The apparatus of claim 11 wherein the primary porous gas distributor comprises a porous polymeric foam.
 19. A resin-wafer electrodeionization (RW-EDI) apparatus for removing carbon dioxide from a gas stream, the apparatus comprising: a cathode; and an anode that is separated from the cathode by a plurality of porous solid ion exchange resin wafers, wherein the cathode anode and wafers are interleaved with ion exchange membranes; the plurality of wafers being arranged in a stack between the cathode and the anode, and comprising one or more basic wafers; each basic wafer comprising a porous basic ion exchange medium, and contacting a primary porous gas distributor across an inlet end of the basic wafer; each primary porous gas distributor having an average pore size in the range of about 100 to about 600 micrometers, and being adapted to introduce a CO₂-containing gas into an aqueous fluid as gas bubbles and disperse the gas bubbles and fluid laterally throughout the distributor; and the basic ion exchange medium within each basic wafer being adapted to convert CO₂ from the gas into bicarbonate ion, and vent a CO₂-depleted gas therefrom; wherein in use, bicarbonate-containing fluid flows out of the apparatus as a concentrated bicarbonate ion solution, and the basic pH of the basic ion exchange medium is maintained by application of an electric potential across the anode and cathode.
 20. The apparatus of claim 19 wherein the basic ion exchange medium contains a carbonic anhydrase enzyme or inorganic catalyst to facilitate conversion of gaseous CO₂ in bicarbonate ion.
 21. The apparatus of claim 20 wherein the carbonic anhydrase enzyme or inorganic catalyst is chemically bound to the basic ion exchange medium thereof to facilitate conversion of gaseous CO₂ into bicarbonate ion.
 22. The apparatus of claim 21 wherein each wafer has a thickness in the range of about 1 mm to about 20 mm.
 23. The apparatus of claim 19 further comprising an intermediate porous gas distributor having an average pore size in the range of about 100 to about 600 micrometers between the inlet end and an outlet end of each basic wafer, dividing the basic wafer into two portions.
 24. The apparatus of claim 23 wherein the primary porous gas distributor and the intermediate porous gas distributor each comprises a porous polymeric foam.
 25. The apparatus of claim 19 wherein the primary porous gas distributor comprises a porous polymeric foam. 